One of my ideas for an alternate history is with Hadrian not taking over, but some more vigorous leader and then that guy pushing forward, taking over the Parthian Empire, spreading all around the Caspian and Black Seas and putting the Dnieper-Dvina rivers as the frontier of the Empire
If they had done this, they would have crushed the Germanic peoples before they even came to the Roman Empire -- in the 2nd century the Germanics were not very much differentiated into the Eastern, Western and Northern Germanics -- their languages were still comprehensible.
The Slavs in the 2nd century were still a united bunch -- the South, West and East Slavs were still one grouping, speaking a common language living in the Pripet marshes or northern Ukraine.
With a border on the Dnieper-Dvina line and along the Caspian and Black sea
If they had done this then the Germanic invasion of the 3rd century would never have occured and the Huns, Avars, Cumans etc. would have been headed off -- with early warnings.